Fluid transport systems use a liquid or a gas, usually water or air, as a carrier to move objects contained within the fluid. Such systems are often used in a manufacturing environment to transport bulk product or individual workpieces through the manufacturing line. Specifically, in the manufacture of silicon wafers, a water transport system moves silicon wafers through sections of the manufacturing line on a water track where jets of water propel the wafers to the desired destination. Other water tracks may be designed to carry wafers along a gently sloping flume much like leaves on a stream.
Occasionally, due to turbulence, eddies, or some other discontinuity in the water flow, there may be a section of the fluid transport system where a wafer may reverse direction from time to time. Such reversal may create jams in the manufacturing line, cause damage to the product, or both.
During the processing of silicon wafers in the manufacture of integrated circuit chips, a chemical mechanical polishing (CMP) step is often required to planarize the wafer surface before depositing successive chip layers. This step is required because the chip surface is often uneven following an etch or deposition step, whereas an even surface is preferred for deposition of the next layer. Therefore, a dielectric coating is applied to the chip, and the dielectric coating is then polished to a perfectly planar surface prior to deposition of the next layer.
Polishing occurs in a polisher, where a wafer is held on a wafer holder plate, by suction, and is rotated. A polishing cloth is pressed with a predetermined force onto the wafer, and a stream of abrasive slurry is sprayed on the wafer. The surface of the wafer is thus polished with the polishing cloth and the abrasive slurry until the surface reaches the desired gloss. Because the abrasive slurry leaves a residue on the wafer, a water track is a desirable way to transport the wafer without having to handle the wafer in a way that risks the abrasive slurry residue causing unintentional damage. It is important, therefore, to keep the wafer wet during its transport and to avoid any extra handling which might unintentionally grind the abrasive slurry against the surface of the wafer.
Thus, a water track is often provided to transfer polished wafers partially covered with abrasive slurry residue from the polisher into an unload cassette. The unload cassette is located in a water reservoir filled with water to the same level as the water track. The unload cassette typically has a plurality of empty slots for receiving wafers, and it indexes automatically after filling a slot with a wafer, thus positioning a new empty slot to receive a new wafer. When the cassette is filled with wafers, an alarm alerts an operator to activate the indexing mechanism to index the cassette into a removal position. The operator then removes the cassette and takes it to a cleaning operation where the abrasive slurry is completely cleaned from each wafer.
Specifically, in the wafer polishing section of a wafer manufacturing line, wafers leave the polisher on a water track that carries them to the unload cassette. The unload cassette slot receives the wafer immediately as it leaves the water track. Occasionally, however, a wafer reverses direction out of the cassette and partially back into the water track. When the unload cassette indexes, it then shatters the wafer caught between the cassette and the edge of the water track.
It is thus necessary to provide some way to prevent a wafer from reversing direction once it leaves the water track, so that it does not suffer damage when the unload cassette indexes. Other manufacturing processes having fluid transport systems may have similar needs for prevention of damage to the products or just to keep products flowing in the desired direction along the fluid transport system.
Various mechanisms for changing direction of wafers in a fluid transport system or for otherwise actively moving wafers from one part of a fluid transport system to another are known. Specifically, various systems for transporting wafers between the polisher and the unload cassette are known. U.S. Pat. No. 5,226,758, issued to Kohichi Tanaka et al. and assigned to Shin-Etsu Handotai Co., Ltd., Japan, discloses and claims a semiconductor wafer handling apparatus and method for transferring a wafer from a wafer holder to a wafer cassette submerged in water. Essentially, Tanaka et al. address the same operation of moving wafers from the polisher to the unload cassette as is involved in the present invention. Tanaka et al. claim an apparatus, however, that receives each wafer at the end of the water track and swings 180.degree. to flip each wafer upside down and drop it into the water reservoir containing the cassette. The wafer then sinks and is forced into the receiving slot of the cassette by an unclaimed mechanism, such as a forced water stream.
The process of flipping each wafer entails extra mechanical handling and motion, such extra handling always having associated with it some risk of mechanical failure or mishandling. Such extra handling is not necessary in a fluid transport system having a water track at the same water level as the reservoir, where the wafer can proceed directly from the polisher along the water track and into the unload cassette without any reorientation. Such a system has the disadvantage, however, that a wafer may occasionally drift back out of the unload cassette, thus exposing it to damage when the unload cassette indexes.
There remains a need, therefore, for a solution to the problems identified above. To overcome the shortcomings of the conventional devices, a new unidirectional gate between interconnecting regions of a fluid transport system is provided. An object of the present invention is to provide a flexible, inexpensive gate that enables passage of the wafer into the unload cassette, but keeps the wafer from reversing direction back into the water track. Such a gate would be applicable to any fluid transport system having a similar interface.